TripAdvisor is being investigated by a competition watchdog in Italy following complaints about "fake" reviews.
The Italian Competition Authority said yesterday it had received complaints from consumers as well as hotel and restaurant owners.
It published a document on its website suggesting TripAdvisor may have published the opinions of people who had not actually been to the places they rated.
It also accused TripAdvisor of not making a clear enough the distinction between information volunteered by travellers and business profiles that hotels and restaurants pay to get published.
The authority has also launched an investigation into online booking websites Expedia and Booking.com, saying their agreements with hotels may prevent consumers from getting a better deal.
TripAdvisor said it "fights fraud aggressively" and is confident in its systems and process, but that it cannot comment on pending investigations.
"Every single review goes through our tracking system, which maps the how, what, where and when of each review," a spokesman said. "This system employs sophisticated algorithms to spot patterns of activity, using best practices from a variety of industries.
"We back that up with a team of over 200 content specialists, who manually investigate every review flagged for inspection by our systems, as well as any reports we receive from owners or travellers."
The company said it issued "strong penalties" to deter fraudsters and that the nature of its site allows for inaccuracies to be quickly rectified.
"Nothing is more important to us than ensuring travellers gain an accurate and useful picture of the businesses and destinations they research on TripAdvisor," it added.
"Unfortunately every major service industry has to confront the challenge of fraud, but ultimately, if people didn't find the reviews on our site helpful and accurate they wouldn't keep coming back."
In January this year, the Chinese domain of TripAdvisor was criticised after research from an online reputation management company revealed thousands of suspect postings.
When contacted by Telegraph Travel, a TripAdvisor spokesperson confirmed that some reviews, including some from a user who had posted 1361 times since October 2013, had been "removed from the site pending further investigation".
Hospitality chiefs warned recently that hotels and restaurants are being targeted by "blackmailers" who demand free meals and stays in exchange for not writing bad reviews.
The Telegraph, London